FIELD OF THE INVENTION
The invention pertains to an immersion system, in particular for a magneto-optical storage device with an immersion lens. More specifically, a lens is provided in the system which is essentially in the shape of a spherical segment with a planar face and a curved face and which is composed of a material that is transparent to electromagnetic radiation of the wavelength .lambda. (the radiation used in the storage device) and which has a first refractive index n, greater than that of air.
In prior art magneto-optical storage devices, such as hard disks, immersion systems are used in the form of a single hemispherical immersion lens. The solid immersion lens is composed, for example, of glass or zirconium oxide and consequently has a refractive index n that is greater than that of air. The electromagnetic radiation which is injected into the immersion lens and which is incident radially into the immersion lens and is concentrated at the center point of the sphere, consequently has the wavelength .lambda./n within the lens. .lambda. represents the wavelength in air. The planar face of hemispherical immersion lens is, during the operation of the storage device, at such a small distance (for example approximately 50 nm) from the surface of the storage medium that the medium between the immersion lens and the storage medium, generally air, has no physical effect on the radiation with the wavelength .lambda./n emerging from the immersion lens. For this purpose, this distance is smaller than .lambda./n6. Consequently, there is physical contact between the immersion lens and the storage medium.
The configuration described above has the effect of ensuring that the radiation, for example monochromatic light of a laser, is transmitted with the wavelength .lambda./n (=wavelength within the immersion lens) onto the storage medium. The spot of light on the storage medium is consequently reduced in size in comparison with a system without immersion lens in which the spot of light is approximately (.lambda.).sup.2 to approximately (.lambda./n).sup.2, giving rise to an increased storage density on the storage medium.
For a given wavelength .lambda. of the electromagnetic radiation, the storage density can thus be influenced by means of the refractive index n of the immersion lens.
In known immersion lenses made of glass (for example quartz glass) or zirconium oxide, the refractive index n lies in the range 1.5.ltoreq.n.ltoreq.2.